This invention relates to wireless local area networks for communication between computers or other terminals.
Local area networks (LANs) typically provide for communication between computer terminals located in the same room, building or complex of buildings by electronic connection between the terminals with conductive wires or optical fibers. Wireless communication between terminals using infrared (IR) light emitting diodes (LEDs) is known but suffers from several drawbacks. One drawback of IR LEDs is a lack of connectivity at distances over perhaps fifty feet that is caused by reduction of signal intensity to power levels inadequate to distinguish over ambient IR noise. Another drawback is that of crosstalk between terminals, so that information transmitted from a terminal is not clearly received by another terminal.
Some prior art systems for infrared communication use mirrors to deflect the light between stations. In U.S. Pat. No. 4,017,146, Lichtman teaches angular distribution of a concentrated laser beam by deflecting the beam with a moving mirror so that the beam raster scans over a large angle. The beam impinges upon a given spatial location in short pulses that amount to a small fraction of a given time period, the remainder of the time period being essentially devoid of the beam at that location, and thus the transmission of data being similarly limited. U.S. Pat. No. 4,982,445 to Grant et al. teaches a laser beam communication system for spacecraft utilizing mirrors positioned in the path of the beams which adjust reflection to different angles.
Other systems employ high and low data transmission channels. U.S. Pat. No. 5,321,542 to Frietas et al. teaches an optical communications system utilizing a high bandwidth, high speed infrared data channel along with a more robust, low bandwidth, low speed infrared channel for maintaining communication when the high speed channel is obstructed. U.S. Pat. No. 5,229,593 to Cato teaches a free space laser communication system operating at a high power level for optimum data transmission when a path between terminals is not blocked, and operating at a lower, eye-safe power level when the path between terminals is obstructed.
Still other systems have characteristics that depend upon the medium of transmission. U.S. Pat. No. 5,227,908 to Henmi teaches an intensity modulated infrared signal for improved noise reduction transmission via an optical fiber. U.S. Pat. No. 5,181,135 to Keeler uses light sources tuned for minimizing losses in an under-water communications system. U.S. Pat. No. 5,159,480 to Gordon et al. teaches a communication system for naval vessels that sends out a horizontally dispersed, vertically concentrated infrared signal for receipt by a remote receiver.
Despite these advances in free space communication, certain obstacles remain. Some known systems utilize a form of time multiplexing to avoid confusion between signals from different terminals, thus cutting into the time available for data transfer between separate terminals. Similarly, the frequency with which infrared diodes can be modulated also can limit the speed with which data can be transferred. Moreover, detection of the signals is often thwarted by ambient infrared noise. Furthermore, a free space local area network including terminals disposed in separate rooms has difficulties caused by walls separating the rooms. The term xe2x80x9cfree spacexe2x80x9d is meant to signify that a path through air is available between terminals, although the path need not necessarily be direct. For the situation in which walls substantially seal one room from another, a free space path is not present.
It is an object of the present invention to overcome the aforementioned obstacles.
The above object is accomplished by providing a wireless local area network of separate terminals each of which has a connected transmitter and receiver. The transmitters include a laser diode with an angularly dispersed, narrow bandwidth infrared output, and the receivers have the capability to detect infrared radiation at frequencies emitted by the transmitters. Each terminal can send data to separate terminals by modulating the output of its connected transmitter, and each terminal can receive data from a separate terminal by demodulating radiation detected by its connected receiver.
To avoid confusion between signals of different terminals, the infrared carrier wavelengths transmitted by different terminals can be mutually exclusive, i.e. wavelength diverse. This wavelength diversity allows an increase in the time during which data can be transmitted and received, since signals can be sent simultaneously between various transmitters and receivers rather than by time multiplexing the signals so that only one transmitter and one receiver are actively communicating at any given time. The increase in signal time afforded by wavelength diversity becomes more pronounced as the number of terminals in a network are increased.
The speed of data transmission between terminals can alternatively be increased by increasing the speed with which the transmitters can be modulated. In this regard, laser diodes can be modulated at a much higher frequency than the light emitting diodes that are usually employed for wireless networking. By employing amplitude modulation of laser diodes, the modulation frequency can be in excess of a gigahertz. Due to this high modulation frequency, the terminals can be time multiplexed rather than wavelength diverse, using a time multiplexing protocol such as that offered under the trademark xe2x80x9cEthernetxe2x80x9d. High power output can be achieved with a low modulation current by modulating a fraction of the gain region, such as with a master oscillator power amplifier (MOPA) or flared resonant cavity laser diode.
The output signal from each transmitter is broadcast over a large area at an intensity which can be detected by various receivers. This can be accomplished, first, by limiting the bandwidth of the transmitted signal, to facilitate detection of the signal over ambient noise. Second, the laser diode can be fashioned to have a high output power that is broadcast over a large area so that the intensity at any localized area is eye-safe, by employing lasers with flared outputs for high power, angularly dispersed beams. A dispersive lens can also be disposed in front of the laser output with associated safety switches that terminate the output upon removal of the lens. Alternatively, a fiber coupled laser diode or laser bar array coupled to an array of fibers which are later either bundled or dispersed in space allow for safe levels of light output intensity. A novel laser diode having opposed flared outputs can be used for increased angular dispersion of the output. Output dispersion over a broad solid angle is generally desirable for the networking of the present invention, a broad solid angle being defined in this application as at least 45xc2x0 in any direction across the beam that includes its axis.
For the situation in which free space transmission between terminals located in a single room is not possible, transmission schemes based upon various net-working geometries are presented. For common corporate environments that have large rooms that are subdivided into cubicles by partitions that extend partially to the ceiling to provide individual work rooms, transceiving terminals mounted on the ceiling are employed to ensure data transmission between terminals in separate cubicles. Other buildings may have individual rooms but a mostly common crawl space or attic above most of the rooms, and ceiling mounted transceiving terminals can again be employed, which communication between those transceiving terminals occurring via the crawl space. Still other buildings with individual rooms are connected by doors with access to common hallways, and transceiving terminals can be mounted adjacent the doors, allowing communication via the hallways.